Some hydraulic control systems employ a regeneration circuit to fill the expanding side of a hydraulic actuator with fluid exhausted from the contracting side of the actuator. Thus, less fluid is required from the system pump thereby allowing the fluid from the system pump to be used for other work circuits of the system. One such regeneration circuit is disclosed in U.S. Pat. No. 4,028,889.
One of the problems encountered with such regeneration circuit is that some of the components causing regeneration have heretofore been incorporated within the directional control valve while other components are disposed in the return line between the directional control valve and the tank. Locating the regeneration components at such locations drastically reduces the efficiency of the regeneration circuit. For example, the fluid exhausted from the actuator must travel the full length of the actuator lines between the actuator and the directional control valve, pass through the directional control valve in a first direction and then in a reverse direction, and then travel through the full length of the other actuator lines to the expanding side of the actuator. The shape of the passages through the valve body and the flow control element therein restricts fluid flow therethrough thereby generating a pressure drop in the exhausted fluid. An additional pressure drop is generated due to the fluid having to travel through the actuator lines, which on some vehicles can exceed 7 or 8 meters. The combined affect of the higher pressure drops necessitates the pressure setting of the regeneration circuits to be at a higher level to adequately provide regeneration. U.S. Pat. No. 5,220,862 solves this problem somewhat by mounting the components of the regeneration circuit directly to or in close proximity to the hydraulic actuator to minimize line losses or pressure drops associated with having the fluid travel through long lines or conduits connecting the directional control valve to the actuator. Mounting the regeneration components at that location lets the exhausted fluid go directly to the expanding side of the actuators and bypasses the directional control valve which would generate an additional pressure drop in the regeneration fluid.
One of the disadvantages with such circuit is that the regeneration circuit is primarily used for diverting fluid exhausted from the head end to the rod end of the actuator. Since the volume of fluid needed to fill the rod end is less than the volume of fluid exhausted from the head end, the main directional control valve must be moved to an operating position so that excess fluid exhausted from the head end can pass therethrough to the tank. However, since the timing of the metering slots of most directional control valves are typically designed to provide acceptable operation under pump-to-cylinder meter in operating conditions, fine control of retracting the actuator is difficult particularly if the actuator is supporting a heavy load.
In view of the above, it would be desirable to have a regeneration function combined with a meter-out function in such a way that the meter-out and regeneration function can operate in combination with the main directional control valve for normal retraction of the actuator or can be operated independently of the directional control valve for lowering a gravity load supported by the actuator.
The present invention is directed to overcoming one or more of the problems as set forth above.